1. Field of the Invention
The invention relates to an integrated circuit arrangement with an active filter as well as a method for tuning an active filter.
2. Description of the State of the Art
Integrated circuit arrangements with an active filter are known comprising transconductance stages, each of which can be adjusted by means of a bias current which is to be supplied.
The so-called gm/C filters, in which, apart from transconductance stages, capacitors are also arranged, constitute an important category of active filters. Among other things, gm/C filters are used for broadband filtering (anti aliasing filters), in conjunction with equalisers in digital transmission technology, or in continuous time sigma-delta converters. Without feedback, they are generally used in applications with high bandwidths and medium linearity requirements.
The circuit shown in FIG. 1 shows a structure, which in the literature is referred to as a “biquad”, implemented in gm/C technology with the following transmission function A(s):   A  =                    U        out                    U                  i          ⁢                                           ⁢          n                      =                                                      gm              1                        *                          gm              2                                                          C              1                        *                          C              2                                                            s            2                    +                                                    gm                4                                            C                1                                      *            s                    +                                                    gm                2                            *                              gm                3                                                                    C                1                            *                              C                2                                                        =                        ω          0          2                                      s            2                    +                                                    ω                0                            Q                        *            s                    +                      ω            0            2                              with:                s=complex frequency        ω0=pole frequency        Q=pole quality        
In the above arrangement, C1, C2 and gm1, gm2, gm3, gm4 refer to the capacity of the capacitors shown in FIG. 1, or to the transconductance of the transconductance stages shown in FIG. 1. The circuit is an improvement on, or a special design of, a circuit described in Silva-Martinez, José, “High-Performance CMOS continuous-time filters”, Kluwer Academic Publishers, ISBN 0-7923-9339-2 (compare FIG. 4.1. of this publication).
In practical application, biquad filters are important basic modules for forming active filters, since, with a suitable combination of biquad structures, any desired filter characteristic can be achieved and the position of zeroes and poles in the complex s-plane of biquad structures is influenced to a relatively low degree by variations in the electrical properties of the components used. Precise control of the position of zeroes and poles in the complex s-plane is a prerequisite for the designed filter to meet the prescribed specifications. In this regard, variations in the component characteristics due to process fluctuations during the production of the integrated circuit, as well as due to temperature fluctuations during operation of said integrated circuit, are particularly important.
The design and mode of operation of transconductance stages, such as the stages gm1 to gm 4 in FIG. 1, are well known to the average person skilled in the art. In a nutshell, a transconductance stage, also known as an operational transconductance amplifier (OTA), transconductance element or transconductor, is a device for generating a current signal from a voltage signal that has been input. This is shown in FIG. 1a with reference to the transconductance stage gm1. If the voltage present at the input of the stage is designated Uin, and the current flowing at the output of the stage is designated Iout, then the following applies:Iout=gm1*Uin,wherein gm1 designates the so-called transconductance gain or the transconductance of the device. Usually, the transconductance of a transconductance stage is adjusted by means of a bias current (Itun in FIG. 1a) supplied to the stage, wherein the concrete interrelation between the bias current and the resulting transconductance depends on the actual design of the transconductance stage. Within the scope of this invention it is of importance that, with a given design of the transconductance stage, its transconductance changes if the bias current is changed, i.e. gm=gm (Itun).
For the purpose of adjusting the bias currents of transconductance stages of an active filter, known integrated circuit arrangements also comprise a tuning device for tuning the filter, with said tuning device adjusting the bias currents of the individual transconductance stages and thus the individual transconductance values. During this adjustment, the above-mentioned variations in transconductance due to fluctuations in the production process and fluctuations in the temperature can be compensated for. Tuning devices and strategies for automatic chip-integrated adjustment of a filter are known per se. Such a strategy for example consists of measuring the present filter performance characteristics, and subsequently comparing these performance characteristics with a standard (reference), and subsequently determining a deviation between the present performance characteristics and the reference, and finally calculating a correction signal and supplying it to the filter. By iterative implementation of this method, deviations (errors) can be reduced. To avoid any impairment of filter operation, the present filter performance characteristics can be measured indirectly, at a replica of the filter or parts of the filter, instead of at the filter itself. To this effect it must only be ensured that the behaviour of the replicated filter or of the replicated filter components corresponds to the behaviour of the filter or of the filter components. This condition is met for replicas which are arranged near the filter, on the same chip.
It is thus advantageous, for the purpose of adjusting the filter, to define bias currents for transconductance stages on replicated filters or filter components, with said bias currents subsequently being supplied to the transconductance stages of the filter, either directly or indirectly by means of current mirrors which are known per se.
In FIG. 1, the tuning device for adjusting the transconductance stages is not shown.
If in the filter according to FIG. 1, one takes into account the output resistance, which in practical application exists for every transconductance gm in the form of an output admittance gds (compare FIG. 2), then both the pole frequency and the pole quality change. The filter characteristic or transmission function A(s) no longer corresponds to that where the output resistance has not been taken into account; a situation which generally speaking is disadvantageous.
Known remedies include increasing the output resistance of the transconductance (gain boosting) or implementing negative output resistance which corresponds to the output resistance.
Increasing the output resistance by way of gain boosting means using operational amplifiers or transistors in feedback loops. In the case of high frequencies, the effect of these loops is limited. Furthermore, the output resistance can only be increased, but its effect on the filter characteristics cannot be eliminated.
In theory, the use of negative resistance can fully compensate for the output resistance. However, implementing the very small regulatable transconductance which is necessary for this is very difficult. Furthermore, additional tuning is required in order to regulate the negative resistance to be equal to the output resistance by way of fluctuations in temperature and process. Moreover, the negative resistance implemented by a transconductance exposes the circuit nodes to undesirable capacitance.